The invention relates to a counter-flow heat exchanger with at least one helical tube bundle forming a closed helical flow channel between a core tube and a tubular shell around a common longitudinal axis, a plurality of helical tubes being coiled in a corresponding helical plane with constant pitch around said common axis, arrayed without interruption and combined in the helical tube bundle, while a primary fluid and a secondary fluid respectively traverse the helical tubes and the helical flow channel in countercurrent flow.
It is known that counter-flow heat exchangers provide a higher efficiency than cross-flow type heat exchangers and are particularly necessary when the temperature differences between the heat exchange media are relatively small.
Conventional heat exchangers with a plurality of tubes have drawbacks with regard to the connection and cleaning of numerous inaccessible tubes with small spacing. It is further difficult in most cases to maintain a plurality of relatively long and flexible tubes with the required spacing. Such heat exchangers are in general unsuitable for operation as a counter-flow heat exchanger with little maintenance so that they are unsuitable for various applications.
Plate heat exchangers are on the other hand suitable as counter-flow heat exchangers, but have the disadvantage that they must be dismantled each time to enable cleaning of the plates, while sealing after each dismantling may be problematic.
In a counter-flow heat exchanger of the initially mentioned type, the helical tubes must be arrayed without interruption in order to form a closed helical flow channel and to thereby ensure operation in true contercurrent flow with high efficiency.
However, the assembly of tube bundles with contiguous helical tubes and their connection become particularly problematic as the number of tubes increases and were hitherto at best possible with a very small number of helical tubes.
The related problems underlying the present invention may be explained by, among other things, the fact that coiled tubes can not be formed so exactly as to be arrayed to closed helical surfaces, without at the same time causing stresses. However, the effect of stresses on round tubes in contact along a spiral line is such that with slight disturbances one helical tube may slip off the other. The convex tube walls pressing against each other are consequently in unstable equilibrium, so that a small deflecting force may bring unsecured tubes out of their correct position.
On the other hand, in another known design, helical tubes are kept separated from each other with a given spacing by various support structures. In this case a primary fluid flows through the helical tubes, while a secondary fluid flows essentially across and between these helical tubes. This arrangement thereby essentially corresponds to a cross-flow type heat exchanger with a considerably lower efficiency than a conter-flow heat exchanger.
The following patent publications may be cited to illustrate the invention with regard to the state of the art relating to heat exchangers of the initially defined type: GB-A- 791843; FR-A 2482717; and FR-A- 2214093.
These known heat exchangers have tube bundles with only three to four helical tubes and with a limited exchange surface and a scope of application which is limited accordingly.
As already mentioned, the manufacture of tube bundles of this type becomes particularly problematic when the number of tubes is increased inasmuch as the connection of the contiguous tubes becomes particularly difficult due to the inaccessibility of the tube ends and therefore is not possible with conventional connecting means.
It is further particularly difficult to bend rigid tubes into exactly contiguous coils and to connect them by conventional connecting means.
On the other hand, relatively flexible tubes can be much more easily coiled, but must be secured in their desired position in the tube bundle, in order to obtain stable tube bundles.
The production of compact counter-flow heat exchangers with as large a number of helical tubes as possible would be particularly advantageous in order to obtain a maximum exchange surface with optimum efficiency.